BACKGROUND ART There are numerous technologies and physics principles which have been used for detecting and measuring the size of defects in metal structures. For ferromagnetic metals, magnetic flux leakage techniques and ultrasonic testing have evolved as the most useful procedures in practice.

Although there is considerable overlap in their areas of application, magnetic flux leakage is of most use in measuring significant metal loss, for example due to corrosion or gouging, and ultrasonics has its main application in measuring cracks.

For conventional ultrasound transducers, a liquid is needed to conduct the ultrasound into the component under test. In many situations, however, the use of a liquid is inconvenient, and alternative transducer designs are required, of which some of the most important types are electromagnetic acoustic transducers (hereinafter referred to as EMATs) which directly excite ultrasound in the component or structure under test and do not require an

intermediate medium to convey the sound from the transducer.

The main advantages of EMATs are in providing ultrasonic inspection without liquid or physical coupling, and the ability to excite several specific wave modes that are beneficial for inspection which cannot be excited by conventional piezoelectric transducers.

Disadvantages of EMATs include high electrical power requirements and physically large transducers, because of the low energy conversion efficiency compared to piezoelectric variants of similar inspection performance.

A further disadvantage is that practical transducers designed to transmit ultrasound in a direction parallel to the surface of a component under test produce broad divergent beams of ultrasound, or compensate for this by using long tone pulses. In both cases the system resolving power is reduced, and hence the ability to measure and discriminate defects is reduced.

One known method for improving the quality of ultrasonic inspection is to use a phased array of transducers. This is a system in which a group of transducers are fired with a specific time delay relative to each other and relative to their spatial positions, so as to transmit the ultrasound within a narrower angular range or within pulses of shorter duration in the component

or structure under test. A similar principle applies to receive transducers, where the signals from a number of transducers are combined in a time delay fashion so as to enhance the information content.

However, conventional systems to produce the benefits of a phased array suffer from the disadvantage of excess complexity. Each transducer in the array is separately powered by a drive circuit each producing a similar high frequency pulse train but displaced in time by an amount which matches the physical displacement of the transducers and the velocity of sound in the component or structure under test. In situations where there is a strict limit on space and/or power, this can restrict their application.

US-A-4295214, US-A-4408493 and US-A-4434663 each discloses an EMAT together with an external driver circuit, the EMAT incorporating a pair of coils fed by the external driver circuit to produce signals having a pre-set phase shift of typically 90°.

SUMMARY OF THE INVENTION It would be desirable to be able to provide an electromagnetic acoustic transducer incorporating the advantages of a phased array but without the inherent complexities and bulk of the known arrangements.

According to the present invention there is provided an electromagnetic acoustic transducer for exciting

ultrasound in a material under test, the transducer comprising at least one magnet positioned so as to apply a magnetic field to the material under test, and a plurality of electrical windings spaced along a direction parallel to the surface of the material, adjacent windings being interconnected by an associated capacitance, and a source of alternating current supplying one of the windings with a pulsed signal, the arrangement being such that the pulsed signal is fed from the one winding to the next adjacent winding, and subsequently sequentially to each next adjacent winding, with a time delay determined by the capacitance between adjacent windings, and whereby each winding creates an ultrasound pulse within the material under test, the spacing of the windings and the electrical characteristics thereof being such that the directional characteristics and time duration of the ultrasound wavefront within the material under test is enhanced.

Thus it will be appreciated that the arrangement of windings and capacitances effectively constitutes a ladder network such that, when a single high frequency pulse is applied at one end of the network, the pulse is transmitted from winding to winding with a time delay, determined by the inductance/capacitance characteristics, which matches the propagation speed of the ultrasound in the material under test, whereby the benefits of a phased array are

achieved using a single drive, and without the necessity for active phase delay circuits.

The windings may comprise, for example, C-coils, pancake coils or meander coils, while the transducer may be arranged to transmit an ultrasound pulse at any angle to the surface of the material under test including parallel to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of part of a horizontally polarised shear wave transducer according to the invention, and Fig. 2 shows part of an alternative transducer according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 of the drawings, a component under test, which may be the ferromagnetic defining wall of a gas pipeline, is shown at 2. The transducer of the invention includes a permanent magnet 4 the longitudinal axis of which extends along the length of the component 2, opposed longitudinal faces of the magnet 4 being of opposite polarity. In the illustrated example, the south pole face of the magnet 4 abuts or lies closely adjacent the component 2.

Arranged along the length of the magnet 4 are a plurality of equi-spaced C-cores 6 on each of which is

wound an associated coil 8, the coil system being such as to induce high frequency magnetic fields and eddy currents in the component 2 which interact with the magnetic pattern created by the magnet 4 to produce magnetostrictive or Lorentz or other forces.

More particularly, a coil 8 is connected to the next adjacent coil by means of a capacitor 10 to create a ladder network or delay line whereby each pair of coil 8 and capacitor 10 conveys to the next pair a delayed high frequency signal. The system is fed by a source providing a high frequency (typically 1MHz) pulse, the physical displacement of the cores/coils 6/8 being such that they form a natural phased array. Thus, when the high frequency pulse is applied to one end of the ladder network, the first core 6/coil 8 starts the ultrasound in the component 2. The first core 6/coil8 also passes the pulse to the next core 6/coil 8 with a time delay determined by the inductance and capacitance of the core 6/coil 8/capacitor 10 combination and which matches the propagation speed of the ultrasound in the component 2. This process continues down the ladder network enhancing and shaping the ultrasonic pulse in the component 2 whereby the benefits of a phased array are realised with a single pulsed drive.

Thus the transducer of the invention combines the functions of generating the ultrasound, shaping the pulse and controlling the phase time relationship.

The illustrated arrangement exemplifies a simple linear array of coils displaced from one another by an amount related to the acoustic signal wavelength, and with an inductance/capacitance delay related to the acoustic frequency, whereby an enhanced horizontally polarised shear wave ultrasound pulse is transmitted at an angle to the surface of the component under test or parallel to the surface of the component if required. Such an arrangement thus transforms the typical slow build up tone burst of conventional horizontally polarised shear wave EMATs into a sharp pulse with better inspection characteristics, and achieves this in a much more compact and economic manner than heretofore.

The precise construction of the transducer can vary from that described and illustrated providing the electrical characteristics together with the physical displacements of the cores, coils and capacitances continue to form a natural phased array. In particular, the C-core coils may be replaced by pancake coils or meander coils, while there may be, for example, a plurality of longitudinally aligned magnets, adjacent magnets having opposite poles abutting one another.

Fig. 2 shows such an alternative arrangement in which a plate material 20 under test has a magnetic field applied thereto in the direction of arrow F'by a standard yoke (not shown), the transducer incorporating a plurality of parallel, thin support plates 22 each carrying a plurality of interconnected pancake coils 24 and associated capacitors 26, the components on adjacent plates 22 being interconnected as shown to produce a ladder network or delay line equivalent to that detailed with reference to Fig. 1.

Each plate 22 carries a ground plane conductor 28, adjacent conductors 28 being interconnected as shown to comprise a continuous length, while a terminating resistor is shown at 30.

Other variations and modifications will be apparent to those skilled in the art. The described transducers may comprise either transmit or receive devices, in the latter case the vibration of the component under test causing the magnetic pattern to induce high frequency signals in the coils, while the material under test may be any electrically conductive material.